"Over half of all UFO reports from the late 1950s through the 1960s were accounted for by manned reconnaissance flights," says Gerald K. Haines, a historian for the National Reconnaissance Office who studied secret CIA UFO files for an internal CIA study that examined the spy agency's involvement in UFOs through the 1990s.

Why lie about UFOs? "The Soviets could use UFO reports to touch off mass hysteria and panic in the United States and overload the U.S. air warning system so that it could not distinguish real targets from phantom UFOs," Haines says.

If Cold War hysteria seems to be a less than satisfactory explanation, perhaps it is because there really is more to the story.

POPULAR MECHANICS has learned from nonclassified sources that the United States had a serious reason for wanting the public to keep believing that the strange lights in the sky were of unearthly origin. The government kept the UFO myth alive to disguise the embarrassing fact that during the hottest days of the Cold War, America's two most secret intelligence gathering assets–the A-12 and SR-71 spyplanes–flew toward hostile terrain with the equivalent of cow bells dangling from their necks.

The deception of the public began in the early 1950s. It involved the then highly secret, and to this day little-known, A-12. If you think you saw an SR-71 Blackbird at an air and space museum, the odds are you were actually looking at an A-12. The idea for the plane was conceived in 1954 by CIA director Allen Dulles. The objective of this secret program, according to aviation historian Paul F. Crickmore, was to build a spyplane capable of flying higher and faster than the U-2.

The secret development program, which was originally called Project Aquatone, and then Gusto and then Oxcart, led to the first A-12 mockup. It became connected with UFO lore in late 1959 when, according to Crickmore, it was trucked from the famous Lockheed Skunk Works, in Palmdale, California, to Groom Lake, Nevada. (Also known to UFO enthusiasts as Area 51, this formerly secret test site is located about 100 miles north of Las Vegas, Nevada.) Hidden in the desert and surrounded by then active Atomic Energy Commission testing grounds, the A-12 mockup underwent a series of tests to determine and then reduce its ability to deflect and absorb radar signals. The CIA liked what it saw and ordered a dozen.

Lockheed had built what to this day is considered the most amazing aircraft of all time. But before it could fly, it needed engines that could propel the plane to Mach 3.2 and an altitude of more than 97,600 ft. In February 1962, Pratt & Whitney announced its already overdue J58 engines could not be delivered anytime soon. As an interim solution, they offered less powerful J75 engines that, according to Crickmore, would take the A-12 to about 50,000 ft. and a speed of Mach 1.6. CIA engineers accepted the offer after calculating that an A-12 equipped with a pair of J75 engines should be able to fly faster than Mach 2. The radar-deflecting shapes of the F-117A (top) and SR-71 (above) lend themselves to misinterpretations as UFOs.

"In order to placate the directors who controlled the agency's purse strings, [Lockheed test pilot] Bill Park dived an A-12 to Mach 2," says Crickmore. "[It] relieved some of the high-level pressure on the design team." Without intending to, Park also opened a new chapter in UFO history.

One of the features about UFO sightings that has consistently baffled the experts is their apparent ability to swoop downward, hover and then soar into the sky at impossible speeds.

Viewed head on, this is exactly how an A-12 or an SR-71–its J58-powered successor–appears to move at times during a normal flight. The maneuver is called a "dipsy doodle."

Col. Richard H. Graham, who commanded the U.S. Air Force 9th Strategic Reconnaissance Wing and has written a history of the SR-71 titled SR-71 Revealed, recently explained the dipsy doodle to PM. The pilot begins by climbing to about 30,000 ft. with the afterburners glowing. At about 33,000 ft., with the plane at Mach .95, he noses the aircraft over. Heading down at a pitch as great as 30 degrees, the plane falls as fast as 3000 ft. per minute. After 10 to 20 seconds, the pilot pulls out of the dive, then accelerates skyward at more than twice the speed of sound.

There is one more very UFO-like characteristic of the SR-71: The glow of its exhaust periodically turns green.

The SR-71 burns fuel modified to withstand high temperatures. It doesn't light easily. "One early 'hiccup' was ignition," Crickmore recalls. "The [J58] engine would not start no matter what procedure was tried."

Eventually the problem was solved by the introduction of a chemical that explodes on contact with the atmosphere. Graham says it must be introduced into the engine when it is started, and it also kicks-in the afterburners. This happens after each aerial refueling, which, given the SR-71's enormous thirst, is quite often. Each time, it produces another image that could be misinterpreted as a UFO–flashing colored lights.

The green flash and distinctive dipsy doodle can be spotted from miles away. Observing the pattern created by these strange sights provides a map to the SR-71's target area, giving those on the ground enough time to hide whatever the spyplane has been sent to photograph.

Curiously, the ebb and flow of UFO sightings in the Southwest correspond with the comings and goings of secret aircraft. Some of the most intense UFO spottings coincided with the testing of the F-117A stealth fighter, which was stationed just west of Area 51. These may account for the yet unexplained sightings.

What better way to hide extraordinary aircraft than to wrap them in the compelling fiction of aliens?

"You are only paranoid when you Imagine people are against you"